1. Field of Use
The present invention relates to a synthetic quartz glass member for use in the optical system of a lithographic apparatus employing an excimer laser as a light source and, in further detail, it relates to a synthetic quartz glass member for use in the illumination system or projection system, such as a lens, a prism and a beam splitter of an ArF excimer laser lithographic apparatus.
2. Prior Art
In recent years, along with the increasing degree of integration in LSIs, patterns for integrated circuits drawn on wafers have become finer and mass production has been started for a super LSI in which super fine patterns of quarter xcexcm or less are drawn. For obtaining such a super fine pattern, the wavelength of optical sources for exposing and thereby drawing them have also to be made shorter, and steppers using an excimer laser as a light source have been developed and steppers using a KrF excimer laser (wavelength: 248 nm) as the light source have been put to practical use. Further, steppers using an ArF excimer laser (wavelength: 193 nm) as a light source have attracted attention as steppers for the next generation of apparatus. Quartz glass, fluorite and the like can be mentioned as glass materials showing sufficient transmittance even in a short wavelength region such as of a KrF excimer laser or the ArF excimer laser. Among them, synthetic quartz glass obtained by flame hydrolysis of silicon compounds and the like at high purity into transparent glass is suitable for optical materials to use in lithography using excimer lasers as the light source. Particularly, in the case of using the synthetic quartz glass as the optical material for the ArF excimer laser lithographic apparatus, it is said that a permissible limit for the internal transmittance at 193.4 nm of wavelength is 99.8% as described in Laid-Open Japanese Patent No. 53432/1998. Therefore, in this publication, the Na concentration is defined as 20 wt.-ppb or less. The synthetic quartz glass at the Na concentration of 20 wt.-ppb or less has been produced by a process under severe control for the synthesis conditions of quartz glass in order to prevent Na contamination by a secondary heat treatment such as a homogenizing treatment. In the ingot obtained under strict setting for the production conditions of the synthetic quartz glass, while the homogeneity in the longitudinal direction (optical axis) can be made higher, it is difficult to make the homogeneity higher in the direction perpendicular thereto (hereinafter referred to as a lateral direction). It is further difficult to remove striae in the lateral direction. They are formed when the conditions change even slightly during growing, so that the synthetic quartz glass ingots have to be produced under extremely strict production conditions. However, fringes in the lateral direction cannot be avoided. This is a phenomenon appearing regularly in the growing direction of the synthetic quartz glass ingots which is usually referred to as a layer or a layered structure and recognized as periodical and fine unevenness of the interference fringe observed upon transmission of a light in a direction perpendicular to the growing direction. The layered structure is a fine periodical structural fluctuation formed upon growing soot or glass by rotating a target for depositing the soot which, although it may differ as to density or thinness, cannot be removed completely by merely setting the production conditions. Then, the layered structure gives significant trouble in the case of using the synthetic quartz glass as a projection system, for example, a beam splitter (as described in xe2x80x9cAPPLIED OPTICSxe2x80x9d, Vol. 31, No. 31, pp. 6658-6661).
As mentioned above, the synthetic quartz glass described in Laid-Open Japanese Patent No. 53432/1998 cannot be used quite satisfactorily as a synthetic quartz glass member for use in ArF excimer laser lithography, particularly, as a synthetic quartz glass member for a projection system required for large size in view of its lack of homogeneity. Accordingly, the secondary heat treatment referred to in this publication is essential also in the synthetic quartz glass described in the publication. However, the homogenizing processing step or molding step described above is conducted at an extremely high processing temperature exceeding 1800xc2x0 C. and the strain removing step also requires a long time for the processing although the processing temperature is relatively as low as about 1100xc2x0 C. Therefore, thermal diffusion of Na takes place from furnace materials, crucibles and atmospheres to contaminate members during the secondary heat treatment to lower the transmittance. Due to the thermal diffusion of Na, the quartz glass causes a Na concentration gradient from the outer surface to the inside in which the Na concentration is higher near the outer surface and lower in the inside. This Na concentration gradient results in a transmittance distribution of the quartz glass and, when a lens is prepared from the synthetic quartz glass member described above for instance, the transmittance lowers at the outer circumference relative to the central part. As a result, not only uniform intensity of transmission light cannot be obtained but also distribution of the refractive index is caused by light absorption making it difficult to use it as the optical material of an exposure apparatus for use in ArF excimer laser lithography.
For using the synthetic quartz glass member suitably as a transmission material of an exposure apparatus for use in ArF excimer laser lithography, it is important to maintain the resistance to laser irradiation, namely, high transmittance and high homogeneity stably for a long period of time. Generally, when an excimer laser is irradiated to quartz glass, paramagnetic defects referred to as Exe2x80x2-center or NBOH-center (NBOHC) are formed and transmittance in a ultraviolet region is lowered since the defects have an absorption band in the ultraviolet region. Further, shrinking of quartz glass accompanied by laser irradiation, referred to as laser compaction, is also observed. The refractive index is increased owing to the shrinkage, thereby worsening the image focusing characteristics of the lens member in exposure apparatus.
It is an object of the invention to provide a quartz glass member for use in an ArF excimer laser having high homogeneity, as well as high transmittance to ArF excimer laser irradiation and also excellent resistance to the laser irradiation.
The invention is based on the finding that a synthetic quartz glass member having high transmittance and high homogeneity, as well as having excellent in the resistance to laser irradiation can be obtained by thermally or mechanically removing layered structures, striae in three directions and internal strains of a quartz glass ingot produced by using a silicon compound of high purity and irradiating with continuous ultraviolet rays at a wavelength of 260 nm or less to a synthetic quartz glass member having an Na concentration of 25 to 60 wt.-ppb.
The synthetic quartz glass member according to the invention not only has excellent homogeneity, but also exhibits high internal transmittance for ArF excimer laser radiation and high resistance against laser radiation and therefore, it is suitable as an optical material for ArF excimer laser steppers. In particular, since even a large member exceeding 200 mm in size has superior homogeneity, as well as exhibits high transmittance for ArF excimer laser radiation, it is useful as a material for lenses, beam splitters and the like for steppers.
The invention for attaining the foregoing object concerns a synthetic quartz glass member for use in ArF excimer laser lithography comprising a high purity synthetic quartz glass and characterized in that layered structures, striae in three directions and internal strains are thermally and mechanically removed, so that the distribution of refractive index (xcex94n) in a plane orthogonal to the optical axis is 1xc3x9710xe2x88x926 or less, the distribution of refractive index (xcex94n) in a plane parallel to the optical axis is 5xc3x9710xe2x88x926 or less, the birefringence is 2 nm/cm or lower, the hydrogen molecule concentration is 2xc3x971017 molecules/cm3 or higher, and the internal transmittance for wavelength at 193.4 nm is 99.8% or higher.
Synthetic quartz glass of high purity may be produced by a direct flame hydrolysis method of depositing fine silica particles produced by flame hydrolysis of a silicon compound, for example, silicon tetrachloride and methyl trimethoxy silane and tetramethoxy silane of high purity, on a target and, simultaneously, vitrifying the compound under melting, or a soot method of depositing fine silica particles once on a target and then heat melting to vitrify in an electric furnace. Then, when the synthetic quartz glass is subjected to homogenization, layered structures, and striae in three directions are removed thermally and mechanically. High homogeneity is applied so that the distribution of refractive index (xcex94n) in a plane orthogonal to the optical axis is 1xc3x9710xe2x88x926 or less, the distribution of refractive index (xcex94n) in a plane parallel to the optical axis is 5xc3x9710xe2x88x926 or less, the birefringence is 2 nm/cm or lower. The homogenization is generally conducted in a refractory furnace while maintaining at a high temperature of 1800xc2x0 C. or higher for a long period of time. However, contamination caused by impurities from furnace materials, jigs and atmospheres occurs during the homogenization, and, particularly, the Na concentration, which remarkably lowers the transmittance of the ArF excimer laser, increased to 30 wt.-ppb or more. Accordingly, it is preferred to use the method of homogenizing treatment described, for example, in European Patent Application EP-A1 673 888 by avoiding contact with a furnace material. Homogenization is a method of supporting a synthetic quartz glass ingot on both longitudinal ends by support members, forming a molten zone in the synthetic quartz glass ingot by a burner while rotating the ingot around an axis connecting the supported ends as a center, applying a pressure in the direction of the supporting axis and protruding outwardly in the molten zone, then supporting the side surfaces thereof on a support and then applying the same homogenization as described above. By this method, the Na concentration contained in the synthetic quartz glass member can be lowered to 20 wt.-ppb or less.
After the homogenization, a molding step is used to form an optical member in which it is formed into a square, cylindrical or prismatic shape of its intended application using the weight of the synthetic quartz glass or by a compulsory force. High temperature processing at 1900xc2x0 C. or higher is used also in this molding, making Na contamination inevitable. Even when a graphite furnace with the ash content for Na concentration being 10 wt.-ppb or less is used, the Na concentration in the synthetic quartz glass is 10 wt.-ppb or more. While the synthetic quartz glass member obtained thereby is subjected to a strain removing step for removing internal strains, the processing requires a long processing time when conducted at a relatively low temperature such as 1100xc2x0 C. or higher and the Na concentration in the synthetic quartz glass is 10 wt.-ppb or more even when an alumina furnace material at a purity of 99% or higher is used. When the homogenization treatment, molding and strain removing processing are applied to the synthetic quartz glass member as described above, at least about 25 wt.-ppb of Na is incorporated. Since Na is actually contaminated by the graphite or the external atmosphere, it is localized at a contact portion or near the surface of the glass material. That is, such a concentration distribution exists that the Na concentration is as high as 25 to 60 wt.-ppb near the surface and it decreases toward the inside of the glass material. Then, when continuous ultraviolet rays at a wavelength of 260 nm or less are irradiated to the synthetic quartz glass member with the Na concentration of 25 to 60 wt.-ppb, a structural change occurs in the quartz glass which results in a bleaching of an absorption induced by the Na content. Due to the bleaching of this absorption, the transmittance of the quartz glass member is increased. It is preferred that ultraviolet rays are irradiated at a higher irradiation dose to a portion nearer to the surface of the member and the irradiation dose decreases toward the inside, the transmittance is improved such that the Na concentration distribution is offset, and the transmittance is improved for the entire member, thereby enabling it to increase the internal transmittance to 99.8% or higher. This processing can improve the internal transmittance up to the range described above even for a large sized member such as a cylindrical shape of 200 mm diameter or more or a prismatic shape with a diagonal length of 200 mm or more for at least one plane.
Lamps for irradiating the continuous ultraviolet rays can include a low pressure mercury lamp with main wavelength of 253.7 nm and 184.9 nm, an Xe2 excimer lamp with a wavelength of 172 nm or a KrCl excimer lamp with a wavelength of 222 nm. Further, the surface roughness Rmax of the synthetic quartz glass member to be irradiated with ultraviolet rays is preferably 30 xcexcm or less. If the surface roughness exceeds 30 xcexcm, scattering of ultraviolet rays is increased and no improvement can be expected for the processing effect. Furthermore, it is preferred that the irradiation dose of ultraviolet rays is at least 1 mW/cm3 and the irradiation time is 50 hrs or more. When the irradiation dose is less than the range described above, there is no irradiation effect, whereas the internal transmittance cannot be improved to a desired level if the irradiation time is less than the range described above.
The synthetic quartz glass member according to this invention is preferably incorporated with hydrogen molecules such that stability can be maintained also for long-term use. The hydrogen molecule concentration is preferably 2xc3x971017 molecules/cm3 or more. Hydrogen molecules can be incorporated at that concentration by optimizing the growing conditions of an ingot in the case of quartz glass produced by a direct flame hydrolysis method. It is preferred to treat a quartz glass body, as required, at a temperature of 1 atm or higher and at a temperature from 600 to 1200xc2x0 C. in a high-pressure hydrogen-processing furnace. Further, in the case of a soot method, the hydrogen molecules within the range described above can be incorporated also during or after sintering, by processing the quartz glass body at a pressure of 1 atm or higher and at a high temperature of from 600 to 1200xc2x0 C. in the high pressure hydrogen processing furnace. By the incorporation of the hydrogen molecules within the range described above, the synthetic quartz glass member according to the invention is stable also to irradiation by ArF excimer laser beams for a long period of time and does not cause compaction or induced suction.
Physical property values for the following examples and comparative examples are values determined by the following measuring methods.
i) Refractive Index Distribution:
Measuring method by a Fizeau interferometer (measured wavelength: 632.8 nm)
ii) Birefringence:
Comparison method by observation with naked eyes relative to a standard inspection plate by a cross Nicol method
iii) Striae:
Observation with naked eyes
iv) Internal Transmittance at 193 nm:
Measuring method determined as (T/90.68)xc3x97100 relative to an apparent transmittance T% at 10 mm thickness, based on 90.68% obtained by subtracting 0.18% known as a loss in a Rayleigh scattering from 90.86%.
v) Na Concentration:
Measuring method by a flameless atomic absorption analysis
vi) Hydrogen Molecular Concentration:
Measuring method as described in V. S. Khottimchenko et al., J. Appl. Spectrosc., 46, 632-635 (1987)